Asymmetric asynchronous mirroring for high availability

ABSTRACT

Maintaining data at a failover site includes an application writing data at a primary site corresponding to the failover site, where data is transferred asynchronously from the primary site to the failover site, the application issuing an application check point and initiating a system check point in response to the application issuing the application check point, where the system check point causes data at the failover site to be consistent with the primary site. The application check point may complete only after the system check point completes. The application check point may complete independently of completion of the system check point. The application check point may complete either only after the system check point completes or independently of completion of the system check point according to a specific criteria.

TECHNICAL FIELD

This application is related to the field of data storage and, more particularly, to systems for managing data sharing over one or more networks.

BACKGROUND OF THE INVENTION

In current storage networks, and particularly storage networks including geographically remote directors (or nodes) and storage resources, preserving or reducing bandwidth between resources and directors while providing optimized data availability and access is highly desirable. Data access may be localized, in part, to improve access speed to pages requested by host devices. Caching pages at directors provides localization, however, it is desirable that the cached data be kept coherent with respect to modifications at other directors that may be caching the same data. An example of a system for providing distributed cache coherence is described in U.S. Patent App. Pub. No. 2006/0031450 to Unrau et al., entitled “Systems and Methods for Providing Distributed Cache Coherency,” which is incorporated herein by reference. Other systems and techniques for managing and sharing storage array functions among multiple storage groups in a storage network are described, for example, in U.S. Pat. No. 7,266,706 to Brown et al. entitled “Methods and Systems for Implementing Shared Disk Array Management Functions,” which is incorporated herein by reference.

Data transfer among storage devices, including transfers for data replication or mirroring functions, may involve various data synchronization processing and techniques to provide reliable protection copies of data among a source site and a destination site. In synchronous transfers, data may be transmitted to a remote site and an acknowledgement of a successful write is transmitted synchronously with the completion thereof. In asynchronous transfers, a data transfer process may be initiated and a data write may be acknowledged before the data is actually transferred to directors at the remote site. Asynchronous transfers may occur in connection with sites located geographically distant from each other. Asynchronous distances may be distances in which asynchronous transfers are used because synchronous transfers would take more time than is preferable or desired. Particularly for asynchronous transfers, it is desirable to maintain a proper ordering of writes such that any errors or failures that occur during data transfer may be properly identified and addressed such that, for example, incomplete data writes be reversed or rolled back to a consistent data state as necessary. Reference is made, for example, to U.S. Pat. No. 7,475,207 to Bromling et al. entitled “Maintaining Write Order Fidelity on a Multi-Writer System,” which is incorporated herein by reference, that discusses features for maintaining write order fidelity (WOF) in an active/active system in which a plurality of directors (i.e. controllers and/or access nodes) at geographically separate sites can concurrently read and/or write data in a distributed data system. Discussions of data ordering techniques for synchronous and asynchronous data replication processing for other types of systems, including types of remote data facility (RDF) systems produced by EMC Corporation of Hopkinton, Mass., may be found, for example, in U.S. Pat. No. 7,613,890 to Meiri, entitled “Consistent Replication Across Multiple Storage Devices,” U.S. Pat. No. 7,054,883 to Meiri et al., entitled “Virtual Ordered Writes for Multiple Storage Devices,” and U.S. patent application Ser. No. 12/080,027 to Meiri et al., filed Mar. 31, 2008, entitled “Active/Active Remote Synchronous Mirroring,” which are all incorporated herein by reference and are assigned to the assignee of the present application.

In an active/active storage system, if there are multiple interfaces to a storage device, each of the interfaces may provide equal access to the storage device. With active/active storage access, hosts in different locations may have simultaneous read/write access via respective interfaces to the same storage device. Various failures in an active/active system may adversely impact synchronization and hinder the ability of the system to recover. Especially problematic are failure scenarios in active/active storage systems involving asynchronous data transmissions.

Accordingly, it is desirable to provide an effective and efficient system to address issues like that noted above for a distributed storage system, particularly in an active/active storage system.

SUMMARY OF THE INVENTION

According to the system described herein, maintaining data at a failover site includes an application writing data at a primary site corresponding to the failover site, where data is transferred asynchronously from the primary site to the failover site, the application issuing an application check point and initiating a system check point in response to the application issuing the application check point, where the system check point causes data at the failover site to be consistent with the primary site. The application check point may complete only after the system check point completes. The application check point may complete independently of completion of the system check point. The application check point may complete either only after the system check point completes or independently of completion of the system check point according to a specific criteria. Data for I/O operations may be performed at the failover site is transferred synchronously from the failover site to the primary site. Maintaining data at a failover site may also include regularly initiating system check points independently of any application check points. Maintaining data at a failover site may also include skipping a regularly scheduled system check point in response to having recently performed a system check point following an application check point. Each of the sites may include at least one host, at least one director, and at least one disk array. The system check point may include providing dependent cache data from the primary site to the failover site. The data may be pulled from the primary site by the failover site.

According further to the system described herein, computer software, provided in a computer-readable medium, maintains data at a failover site when an application writes data at a primary site corresponding to the failover site, the data being transferred asynchronously from the primary site to the failover site. The software includes executable code that determines when the application issues an application check point and executable code that initiates a system check point in response to the application issuing the application check point, where the system check point causes data at the failover site to be consistent with the primary site. The application check point may complete only after the system check point completes. The application check point may complete independently of completion of the system check point. The application check point may complete either only after the system check point completes or independently of completion of the system check point according to a specific criteria. Data for I/O operations performed at the failover site may be transferred synchronously from the failover site to the primary site. The software may also include executable code that regularly initiates system check points independently of any application check points. The software may also include executable code that skips a regularly scheduled system check point in response to having recently performed a system check point following an application check point. Each of the sites may include at least one host, at least one director, and at least one disk array. The system check point may include providing dependent cache data from the primary site to the failover site. The data may be pulled from the primary site by the failover site.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the system described herein are explained with reference to the several figures of the drawings, which are briefly described as follows.

FIG. 1 shows a network configuration of a distributed storage system that may be used in accordance with an embodiment of the system described herein.

FIG. 2 is a schematic illustration showing a system that includes a plurality of data centers in communication via a network that may be used in accordance with an embodiment of the system described herein.

FIG. 3 is a schematic illustration showing a distributed storage system with multiple sites according to an embodiment of the system described herein.

FIG. 4 is a schematic illustration showing failure scenarios in a distributed storage system in accordance with various embodiments of the system described herein.

FIGS. 5 and 6 show alternative configurations of distributed storage systems that may be used in accordance with embodiments of the system described herein.

FIG. 7 is a flow diagram showing a method for providing mobility of a virtual machine between a first site and a second site of an active/active system according to an embodiment of the system described herein.

FIG. 8A is a flow diagram showing a method for using the inter-cluster link (director networks) between director clusters of multiple sites in an active/active system according to an embodiment of the system described herein.

FIG. 8B is a flow diagram showing a method for using the inter-cluster link (host networks) between host clusters of multiple sites in an active/active system according to an embodiment of the system described herein.

FIG. 9 is a schematic illustration of a write order fidelity (WOF) pipeline and visualizes some details according to an embodiment of the system described herein.

FIG. 10 is a flow diagram showing a method of pulling data block ownership and data from the losing site to the winning site according to an embodiment of the system described herein.

FIG. 11 is a schematic illustration of a write order fidelity (WOF) pipeline and visualizes some details that may be used according to an embodiment of the system described herein.

FIG. 12 is a flow diagram showing a method for taking and using synchronized snapshots of the memory state (image) of a VM and the storage state of a disk to provide a fail-over mechanism for high availability.

FIG. 13 is a flow diagram showing processing following a failure in the active/active system according to an embodiment of the system described herein.

FIG. 14 is a flow diagram showing processing provided in connection with coordinating application check points and system check points according to an embodiment of the system described herein.

FIG. 15 is a flow diagram showing processing provided in connection with coordinating application check points and system check points according to an alternative embodiment of the system described herein.

FIG. 16 is a schematic illustration showing a relatively active site and a relatively inactive site according to an embodiment of the system described herein.

FIG. 17 is a flow diagram showing processing provided in connection with selectively coordinating application check points and system check points according to an embodiment of the system described herein.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

FIG. 1 shows a network configuration of a distributed storage system 50 that may be used in accordance with an embodiment of the system described herein. As shown, a plurality of host devices 10 (10 ₁ to 10 _(N)) are communicably coupled with a plurality of directors 20 (20 ₁, 20 ₂ to 20 _(N)). Each of the directors 20 may include a processor (CPU) component 22, such as a microprocessor or other intelligence module, a cache component 24 (e.g., RAM cache), an instance of a distributed cache manager 26 and/or other local storage and communication ports. (In general, “N” is used herein to indicate an indefinite plurality, so that the number “N” when referred to one component does not necessarily equal the number “N” of a different component. For example, the number of hosts 10 may or may not equal the number of directors 20 in FIG. 1.) Cache memory may be considered memory that is faster and more easily accessible by a processor than other non-cache memory used by a device.

Each of the hosts 10 may be communicably coupled to one or more of directors 20 over one or more network connections 15, 16. It is noted that host devices 10 may be operatively coupled with directors 20 over any of a number of connection schemes as required for the specific application and geographical location relative to each of the directors 20, including, for example, a direct wired or wireless connection, an Internet connection, a local area network (LAN) type connection, a wide area network (WAN) type connection, a VLAN, a proprietary network connection, a Fibre channel (FC) network etc. Furthermore, hosts may also be coupled to one another via the networks 15, 16 and/or operationally via a different network 5 and several of the hosts 10 may be clustered together at one or more sites in which the sites are geographically distant from one another. It is also noted that in various embodiments the networks 15, 16 may be combined with the SAN networks 30, 31.

Each of the directors 20 may also include, or be communicably coupled with, one or more file systems, such as a virtual machine file system (VMFS), a new technology file system (NTFS) and/or other appropriate file system, and may be communicably coupled with one or multiple storage resources 40, 41, each including one or more disk drives and/or other storage volumes, over one or more storage area networks (SAN) 30, 31, and/or other appropriate network, such as a LAN, WAN, etc. The directors 20 may be located in close physical proximity to each other, and/or one or more may be remotely located, e.g., geographically remote, from other directors, as further discussed elsewhere herein. It is possible for the SANs 30, 31 to be coupled together, and/or for embodiments of the system described herein to operate on the same SAN, as illustrated by a dashed line between the SAN 30 and the SAN 31. Each of the directors 20 may also be able to intercommunicate with other directors over a network 25, such as a public or private network, a peripheral component interconnected (PCI) bus, a Fibre Channel (FC) network, an Ethernet network and/or an InfiniBand network, among other appropriate networks. In other embodiments, the directors may also be able to communicate over the SANs 30, 31 and/or over the networks 15, 16. Several of the directors 20 may be clustered together at one or more sites and in which the sites are geographically distant from one another. The system described herein may be used in connection with a vSphere and/or VPLEX product produced by VMware Inc. of Palo Alto, Calif. and EMC Corporation of Hopkinton, Mass., respectively. The system described herein may also be used in connection with an RDF storage product produced by EMC Corporation, such as a Symmetrix product. Although discussed and illustrated in connection with embodiment for a distributed storage system, the system described herein may generally be used in connection with any appropriate distributed processing system.

Each distributed cache manager 26 may be responsible for providing coherence mechanisms for shared data across a distributed set of directors. In general, the distributed cache manager 26 may include a module with software executing on a processor or other intelligence module (e.g., ASIC) in a director. The distributed cache manager 26 may be implemented in a single director or distributed across multiple intercommunicating directors. In certain aspects, each of the directors 20 may be embodied as a controller device, or blade, communicably coupled to one or more of the SANs 30, 31 that allows access to data stored on the storage networks. However, it may be appreciated that a director may also be embodied as an intelligent fabric switch, a hub adapter and/or other appropriate network device and may also be implemented as a virtual machine, as further discussed elsewhere herein. Because Locality Conscious Directory Migration (LCDM) is applicable to databases, any suitable networked director may be configured to operate as an access node with distributed cache manager functionality. For example, a distributed cache manager may be run on one or more desktop computers and/or virtual machines with a network connection.

According to the system described herein, a distributed storage system may enable a storage device to be exported from multiple distributed directors, which may be either appliances or arrays, for example. With an active/active storage system, hosts in different locations may have simultaneous write access to mirrored exported storage device(s) through a local front-end thereof (i.e., a director). The distributed storage system may be responsible for providing globally consistent and coherent data access. The system described herein may be used in connection with enabling the distributed storage system to meet consistency guarantees and maximize data access even in response to failures that may cause inconsistent data within the distributed storage system.

Using virtualization software, one or more of the physical servers may be subdivided into a plurality of virtual machines. A virtual machine (VM) is a software implementation of a machine that executes programs like a physical machine. Virtualization software allows multiple VMs with separate operating systems to run in isolation on the same physical machine. Each VM may have its own set of virtual hardware (e.g., RAM, CPU, NIC, etc.) upon which an operating system and applications are loaded. The operating system may see a consistent, normalized set of hardware regardless of the actual physical hardware components. The term “virtualization software” is used herein to generally refer to any and all software that supports the operation of one or more VMs. A number of virtualization software products exist, including the VMware product family provided by VMware, Inc. of Palo Alto, Calif. A benefit of providing VMs is the ability to host multiple, unrelated, clients in a single physical server. The virtualization software may maintain separation of each of the clients, and in which each of the clients separately access their own virtual server(s). Other virtualization products that may be used in connection with the system described herein include Hyper-V by Microsoft Corporation of Redmond, Wash., public license virtualization products and/or other appropriate virtualization software.

Configuring and deploying VMs is known in the field of computer science. For example, U.S. Pat. No. 7,577,722 to Khandekar, et al., entitled “Provisioning of Computer Systems Using Virtual Machines,” which is incorporated herein by reference, discloses techniques for configuring and deploying a VM according to user specifications. VMs may be provisioned with respect to any appropriate resource, including, for example, storage resources, CPU processing resources and/or memory. Operations of VMs may include using virtual machine images. A virtual machine image is the image of the virtual machine as it resides in the host's memory. A virtual machine image may be obtained for an operating VM and transferred to another location where the VM continues execution from the state defined by the virtual machine image. In this way, the virtual machine image may be a snapshot of an execution state of a program by a VM that may be moved between different locations and processing thereafter continued without interruption.

In a virtualization environment, a virtual center, that may be referred to as a vCenter, may provide a central point of control for managing, monitoring, provisioning and migrating virtual machines. Virtual centers may operate to control virtual machines in data centers and in connection with cloud computing including using both internal and external cloud infrastructures and hybrids thereof.

FIG. 2 is a schematic illustration showing a system 100 that includes a first data center 102 in communication with a second data center 104 via a network 106. Although the following embodiments are discussed principally in connection with data centers 102, 104 any number of additional data centers, represented as data centers 102′, 104′, may be also be used in connection with the system described herein. Each of the data centers 102, 104 may include a plurality of storage devices and processors (not shown in FIG. 2) for executing applications using a plurality of VMs. The VMs may be configured using any appropriate server virtualization technology, such as that provided by VMware, Inc. of Palo Alto, Calif., including vSphere. VSphere is a suite of tools offering the ability to perform cloud computing utilizing enterprise-level virtualization products such as VMware's ESX and/or ESXi. VSphere allows multiple VMs to run on any ESX host. Other VM technology may be used including any appropriate VM technology provided by other vendors.

The data centers 102, 104 may contain any number of processors and storage devices that are configured to provide the functionality described herein. In an embodiment herein, the storage devices may be Symmetrix storage arrays provided by EMC Corporation of Hopkinton, Mass. Other appropriate types of storage devices and different types of processing devices may also be used in connection with the system described herein. The data centers 102, 104 may be configured similarly to each other or may be configured differently. The network 106 may be any network or similar mechanism allowing data communication between the data centers 102, 104. In an embodiment herein, the network 106 may be the Internet and/or any other appropriate network and each of the data centers 102, 104 may be coupled thereto using any appropriate mechanism. In other embodiments, the network 106 may represent a direct connection (e.g., a physical connection) between the data centers 102, 104.

In various embodiments, VMs may be migrated from a source one of the data centers 102, 104 to a destination one of the data centers 102, 104. VMs may be transferred from one data site to another, including VM mobility over geographical distances, for example, for reasons of disaster avoidance, load balancing and testing, among other reasons. For a discussion of migrating VMs, reference is made to U.S. patent application Ser. No. 12/932,080 to Meiri et al., filed Feb. 17, 2011, entitled “VM Mobility Over Distance,” which is incorporated herein by reference and is assigned to the assignee of the present application. A product, such as EMC's VPLEX Geo and/or VPLEX Global, may be used to enable the resources of disparate storage systems in geographically dispersed data centers to be federated together and utilized as a single pool of virtual storage. VPLEX Geo or Global provide for data mobility, availability and collaboration through active/active data over distance with the ability to non-disruptively move many VMs.

FIG. 3 is a schematic illustration showing a distributed storage system 200 having multiple sites according to an embodiment of the system described herein. Although illustrated with two sites, Site A 201 and Site B 202, the system described herein may also operate in connection with additional sites. Although components are specifically identified with respect to Site A 201, Site B 202 (or any additional site) may also include the components discussed herein. The sites 201, 202 may include one or more hosts grouped in host clusters 210 a,b, one or more directors grouped in director clusters 220 a,b, and disk arrays 240 a,b. Each host cluster 210 a,b and director cluster 220 a,b may each include software and/or other controllers or interfaces to control or administer operations in connection with described functions of the hosts and directors. In an embodiment, each host cluster 210 a,b may include ESX hosts in a vSphere cluster and director cluster 220 a,b may include directors in a VPLEX cluster. Front end networks 215 a,b may connect through host links to the host clusters 210 a,b and through front end links to the director clusters 220 a,b. One or more back end networks 230 a,b may connect through back end links to the director clusters 220 a,b and through array links to the disk arrays 240 a,b. In an embodiment, the front and back end networks may be Fibre Channel networks. The front end networks 215 a,b allow the hosts (or VMs running therein) to perform I/O operations with the host clusters 210 a,b, while the back end networks 230 a,b allow the directors of the director clusters 220 a,b to perform I/O on the disk arrays 240 a,b. One or more host networks 205, such as vSphere Ethernet networks, connect the ESX hosts in host clusters 210 a,b. One or more director networks 225 connect the directors of the director clusters 220 a,b.

Various types of failures, including network failures within a cluster, may result in behaviors that are further discussed elsewhere herein. It should be noted that the host cluster 210 a,b (e.g., vSphere cluster) may be connected in such a way that VMs can keep their network (e.g., IP, FC, IB) addresses when migrating between clusters (for example, by means of a vLan or an open vSwitch). In an embodiment, VPLEX Geo may be used and configured to expose one or more distributed volumes from both VPLEX director clusters. A VMFS may be created on top of these distributed volumes allowing VMs that migrate between the sites to see the same file system in either site. It is also noted that, as illustrated and according to various embodiments, each site 201, 202 may include redundancies in hosts, directors and links therebetween. It should be noted that the active/active system described herein may also be used in active/passive functioning as appropriate or desired.

I/O access may be provided to distributed volumes in an active/active system with two sites separated by an asynchronous distance. For asynchronous operation, a write operation to cluster at a remote site may be acknowledged as soon as a protection copy is made within the cluster. Sometime later the write data is synchronized to the remote site. Similarly, writes to the remote site are later synchronized to a cluster at the local site. Software or other controllers at the director clusters, such as VPLEX, may present the same image of the data on either cluster to provide a cache-coherent view of the data. In an embodiment, this may be achieved by fetching data that has not yet been replicated between a source and destination site (i.e. “dirty” data; as compared with “clean” data which has been copied and is protected on multiple sites) over the inter-cluster link on an as needed basis. In the background, the controller (VPLEX) may synchronize the oldest dirty data between the clusters.

The above operations may work as long as the inter-cluster network is available. If the inter-cluster link fails, both clusters may contain dirty data that is unknown by the respective remote clusters. As a consequence of this failure, the director cluster may rollback the image of the data to a write order consistent point. In other words, the director cluster may rollback the image of the data to a point where it knows the data that is available on both clusters, or to a time where the write data was exchanged between both sites. The director cluster may also guarantee rollback to an image of the disk or volume that is write order consistent, which means that if the data of a specific write is available on the volume, all data of writes that were acknowledged before (“preceded”) that write should be present too. Write order consistency is a feature that allows databases to recover by inspecting the volume image. As noted elsewhere herein, known techniques may provide write order consistency by bucketing writes in what are called deltas and providing the consistency on a delta boundary basis (see, e.g. U.S. Pat. No. 7,475,207 to Bromling et al.).

Suspend/resume migration processing may involve suspending a VM in the source site and resuming that VM in the destination site. Before the suspended VM is resumed, all dirty data for the affected VMFS may be synchronized from the source VPLEX cluster to the destination VPLEX cluster, and the preference (i.e. “winner” site) for the distributed volume may be changed from the source cluster to the destination cluster. The preference attribute may be related to a VPLEX consistency group that contains one or more VMs. Hence, the VM may be in a consistency group of its own or all VMs in a consistency group may be migrated together. To know when the synchronization of VPLEX's dirty cache is finished, the customer may map the VMFS to a distributed volume.

FIG. 4 is a schematic illustration showing failure scenarios, identified as 1-15, in a distributed storage system 200′ having multiple sites like that of distributed storage system 200 according to various embodiments of the system described herein. Table 1 shows the observed behaviors of the different failure scenarios, as schematically shown in FIG. 4, in connection with suspending and resuming migrations of VMs according to various embodiments of the system described herein. The specific embodiments described in Table 1 are discussed in connection with the use of VPLEX Geo clusters (director clusters 220 a,b) and vSphere clusters (host clusters 210 a,b). It is further noted that in connection with characterizing failure scenarios and identifying an appropriate site as a winner site for continuing operations, a cluster witness node may be used. Reference is made, for example, to U.S. patent application Ser. No. 12/930,121 to Ortenberg et al., filed Dec. 29, 2010, and entitled “Witness Facility for Distributed Storage System,” which is incorporated herein by reference, that providing examples of features and uses of a witness node in a distributed storage system in connection with determining failure conditions. It is noted that conditions and/or user preferences may cause a site to be indicated a preferred site; however, in the event of certain failures, a winner site may be other than the preferred site and may cause re-selection of a preferred site.

TABLE 1 Scenario VPLEX Geo behavior vSphere behavior 1. Array failure VPLEX continues providing access to No change. the distributed volume using the remaining (remote) array. When access to the failed array is restored or the array is replaced, the volume on the affected array will be resynchronized automatically. 2a. Array link failure Directors use all available paths to No change. (redundant) an array. All directors in the affected cluster lose one path to one array, but have one remaining path to the array. 2b. Array link failure See scenario “1. Array failure.” — (non-redundant) 3a. Back end Directors use all available paths to No change. network failure an array. All directors in the affected (redundant) cluster lose one path to all local arrays, but one redundant path between the local directors and the local arrays remains (through the redundant back end network). 3b. Back end All directors lose both paths to all No change. network failure local arrays. Hence these directors (non-redundant) lose access to the local mirror legs of their distributed volumes. Reads to the affected volumes will be served from the remote mirror legs. Writes to the affected volumes succeed on the remote mirror leg and cause the local mirror legs to be marked out- of-date. No rebuilds will be started due to lack of array visibility. 4a. Back end link Directors use all available paths to No change. failure (redundant) an array. One director loses one path to all local arrays present, but one path to these arrays remains. 4b. Back end link One director loses both paths to all No change. failure (non- local arrays. Hence this director loses redundant) access to the local mirror legs of its distributed volumes. Reads to the affected volumes will be served from the remote mirror legs. Writes to the affected volumes succeed on the remote mirror leg and cause the local mirror legs to be marked out- of-date, which will cause other directors to start rebuilding the distributed volumes (which will not succeed as long as hosts write to the distributed volumes through the affected director). 5a. VPLEX director VPLEX protected the user data on The ESX hosts use all available paths failure (redundant) another director. This protection to VPLEX. Under this scenario copy will be take the place of the multiple ESX hosts lost paths to one original copy in VPLEX's cache director, but one or more paths coherence protocol and will remain to one or more unaffected eventually be flushed to the back VPLEX directors. end array. 5b. VPLEX director See scenarios “7. VPLEX cluster — failure (non- failure.” redundant) 6a. VPLEX local Directors use all available paths to No change. communication link communicate to other directors. failure (redundant) Under this scenario one director lost one path to each other local director, but another path to each other local director remains. 6b. Local VPLEX The affected VPLEX director will be No change. communication link expelled, effectively removing the failure (non- export of the distributed volume redundant) from that director. 6c. Local VPLEX A VPLEX director uses all available No change. communication paths to communicate to other network failure directors. All directors lost one path (redundant) to all other local directors, but one path to each other local director remains (through the redundant communication network). 6d. Local VPLEX VPLEX single: one director is VPLEX single: No change. communication expelled and the other keeps VPLEX dual and quad: All local VMs network failure running. go into PDL. VMs have to be (non-redundant) VPLEX dual and quad: the VPLEX restarted manually on the surviving local site is expelled. cluster. 7a. Non-preferred The cluster witness tells the VMs and ESX hosts talking to the VPLEX cluster failure preferred cluster to proceed serving failed, non-preferred cluster see all observed by the I/O. Once the failed cluster comes paths to the VPLEX array disappear cluster witness back online all distributed volumes (All Paths Down). vSphere host will be resynchronized. adapter (HA) does not restart VMs across geography so all these VMs are unavailable until the failed, non- preferred VPLEX cluster comes back online or the user moves the VMs to the surviving, preferred VPLEX cluster manually. 7b. Non-preferred The preferred cluster proceeds VMs and ESX hosts talking to the VPLEX cluster failure serving I/O. Once the failed, non- failed, non-preferred VPLEX cluster not observed by the preferred cluster comes back online see all paths to the storage cluster witness all distributed volumes will be disappear (All Paths Down). vSphere resynchronized. HA does not restart VMs across geography so all these VMs are unavailable until the failed, non- preferred VPLEX cluster comes back online or the user moves the VMs to the surviving, preferred VPLEX cluster manually. 7c. Preferred VPLEX Cluster witness overrules the VMs and ESX hosts talking to the cluster failure preference and tells the non- failed, preferred cluster see all paths observed by the preferred cluster to proceed serving to the storage disappear (All Paths cluster witness I/O. Once the cluster comes back Down or APD in vSphere online all distributed volumes will be terminology). vSphere HA does not resynchronized. restart VMs across geography so all these VMs are unavailable until the failed, preferred VPLEX cluster comes back online or the user moves the VMs to the surviving, non- preferred VPLEX cluster manually. 7d. Preferred VPLEX The non-preferred cluster stops All VMs and ESX hosts see all paths cluster failure not serving I/O. Once the failed, to the VPLEX array disappear (All observed by the preferred cluster comes back online Paths Down or APD in vSphere cluster witness I/O will proceed on both clusters. terminology). 8a. Front end link No change. ESX hosts use all available paths to failure (redundant) the VPLEX array. All ESX hosts lose one path to one director, but one or more paths remain to the VPLEX array. 8b. Front end link No change. ESX hosts use all available paths to failure (non- the VPLEX array. All ESX hosts lose redundant) both paths to one director, but two paths remain to one or more unaffected directors. 9a. Front end No change. ESX hosts use all available paths to network failure the VPLEX array. All ESX hosts and all (redundant) VPLEX directors attached to this back end network lost one or more paths, but other paths remain (through the redundant front end network). 9b. Front end No change. All ESX hosts in one cluster lose all network failure paths to the local VPLEX array. (non-redundant) Depending on the VM's operating system and whether disk I/O is happening, the VM fails or waits for the front end network to return. Failed VMs have to be restarted. 10a. Host link failure No change. ESX hosts use all available paths to (redundant) the VPLEX array. One ESX host loses one path, but one path remains to the VPLEX array. 10b. Host link failure No change. One ESX host loses both paths to the (non-redundant) VPLEX array, resulting in a Persistent Device Loss for all volumes. vSphere HA will restart all affected VMs on another ESX host in the same cluster. 11a. ESX host failure No change. vSphere HA restarts the VMs that (redundant) were running on the failed ESX host on a redundant ESX host. 11b. ESX host failure See scenario “12. vSphere Cluster — (non-redundant) failure.” 11c. VM failure No change. vSphere HA restarts the failed VM on any ESX host in the same cluster. 12. vSphere Cluster No change. Failed VMs have to be restarted failure manually. 13a. Inter-site VPLEX VPLEX sites use all available paths to No change. communication communicate to each other. Under network failure this scenario one inter-site (redundant) communication network remains. 13b. Inter-site VPLEX Preferred cluster: No change. Preferred cluster: No change. communication Non-preferred cluster: Directors in Non-preferred cluster: No change, network failure the non-preferred site suspend I/O. because all VMs are running in the (non-redundant) preferred site. during regular operation 13c. Inter-site VPLEX Preferred or source cluster: No Preferred or source cluster: The VMs communication change that are migrating have to be network failure Non-preferred or destination cluster: restarted manually. The VMs that (non-redundant) Directors in the non-preferred or were running in the preferred site during a migration destination site suspend I/O. keep running. Non-preferred or destination cluster: No change because all VMs are running in the preferred or source site. 13d. Inter-site VPLEX Preferred (or formerly destination) Preferred (or formerly destination) communication cluster: No change. cluster: No change. network failure Non-preferred (or formerly source) Non-preferred (or formerly source) (non-redundant) cluster: Directors in the non- cluster: No change, because all VMs shortly after a preferred or source site suspend I/O. are running in the preferred site. migration 14a. Inter-cluster No change. All ESX hosts can still see each other vSphere network over the redundant inter-cluster failure (redundant) vSphere network. 14b. Inter-cluster No change. ESX hosts in different clusters can no vSphere network longer see each other. VMs remain failure (non- running and can only be vMotioned redundant) to ESX hosts within their current cluster. 15a. Inter-site VPLEX Preferred cluster: No change. Preferred cluster: No change. communication Non-preferred cluster: Directors in Non-preferred cluster: No change, network failure the non-preferred cluster suspend because all VMs are running in the (non-redundant) I/O. preferred cluster. plus inter-cluster vSphere network failure (non- redundant) during regular operation 15b. Inter-site VPLEX Preferred or source cluster: No Preferred or source cluster: The VMs communication change. that are migrating have to be network failure Non-preferred or destination cluster: restarted in the source cluster (non-redundant) Directors in the non-preferred or because the migration fails. The VMs plus inter-cluster destination cluster suspend I/O. that were running in the preferred vSphere network cluster keep running. failure (non- Non-preferred or destination cluster: redundant) during a No change because no VMs are yet migration running in this cluster. 15c. Inter-site VPLEX Preferred (or formerly destination) Preferred (or formerly destination) communication cluster: No change. cluster: No change. network failure Non-preferred (or formerly source) Non-preferred (or formerly source) (non-redundant) cluster: Directors in the non- cluster: No change, because no VMs plus inter-cluster preferred or source cluster suspend are running in this cluster anymore. vSphere network I/O. failure (non- redundant) shortly after a migration

Failures may also occur when a VM is migrated while performing I/O operations. The migration of a VM during I/O operations may be referred to herein as “vMotion” and may be facilitated by a VMware product called vMotion. In a director network failure situation during VM migration, both the source cluster directors and the destination cluster directors may contain dirty data. A similar problem may occur when multiple VMs have to be migrated together because they all access one VMFS volume. In an embodiment, this problem could be alleviated by suspending the restart of the VM on the destination cluster until the director cluster (e.g., VPLEX cluster) cache has been synchronized; however, such operation may cause undesirable delays.

In various embodiments, many of the failure scenarios for VM migration may result in the same behavior as for that of suspend/resume migration failure behavior results. Table 2 describes the failure scenarios where the VM migration (e.g., vMotion) results in different behaviors as compared with the suspend/resume migration processing of Table 1. Note that in Table 2, the identified scenarios from Table 1 are split in two: one for the case where dirty data is on the non-preferred cluster and one where dirty data is not on the non-preferred cluster.

TABLE 2 Scenario VPLEX Geo behavior vSphere behavior 13A. Inter-site Preferred cluster: Directors in the Preferred cluster: No change. VPLEX preferred cluster keep serving I/O. Non-preferred cluster: VMs can no communication Non-preferred cluster: Directors in longer access the VPLEX array. They network failure the non-preferred cluster stop can wait for the storage to return or (non-redundant) serving I/O until the inter-site VPLEX they can be restarted manually at during regular communication network returns. the preferred cluster. operation without dirty data in the non-preferred cluster 13B. Inter-site VPLEX Preferred cluster: Directors in the Preferred cluster: ESX hosts and VMs communication preferred cluster discard some should be restarted to find out what network failure recently written data (to directors in data is discarded by the VPLEX array. (non-redundant) either cluster). VPLEX rolls back to a Non-preferred cluster: VMs can no during regular write-order consistent volume image longer access storage. They will have operation with dirty and suspends I/O. The user resumes to be restarted manually on either data in the non- I/O after restarting all ESX hosts and the preferred cluster or the non- preferred cluster VMs present in the preferred cluster preferred cluster. ESX hosts need to by issuing the “resume-after- be restarted. rollback <consistency group>” command. Non-preferred cluster: Directors in the non-preferred cluster suspend I/O until the inter-site communication network returns and the user resumes I/O to indicate that all ESX hosts and remaining VMs are restarted. Writes to distributed volumes on the directors in the preferred cluster after the inter-site communication failure happened, are present on the directors in the non-preferred cluster. 13C. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13A. Inter-site VPLEX scenario “13A. Inter-site VPLEX network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation during vMotion without dirty data in the non- without dirty data in the non- without dirty data in preferred cluster.” preferred cluster.” the non-preferred VMs of failed vMotions have to be cluster restarted manually. 13D. Inter-site Same VPLEX Geo behaviors as in Same vSphere behaviors as in VPLEX scenario “13B. Inter-site VPLEX scenario “13B. Inter-site VPLEX communication communication network failure (non- communication network failure (non- network failure redundant) during regular operation redundant) during regular operation (non-redundant) with data in the non-preferred.” with dirty data in the non-preferred during vMotion with cluster.” dirty data in the VMs of failed vMotions have to be non-preferred restarted manually. cluster 13E. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13A. Inter-site VPLEX scenario “13A. Inter-site VPLEX network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation shortly after without dirty data in the non- without dirty data in the non- vMotion without preferred cluster.” preferred cluster.” dirty data in the The just vMotioned VMs have to be non-preferred restarted manually. cluster 13F. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13B. Inter-site VPLEX scenario “13B. Inter-site VPLEX network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation shortly after with dirty data in the non-preferred with dirty data in the non-preferred vMotion with dirty cluster.” cluster.” date in the non- The just vMotioned VMs have to be preferred cluster restarted manually. 15A. Inter-site Same VPLEX Geo behaviors as in Same vSphere behaviors as in VPLEX scenario “13A. Inter-site VPLEX scenario “13A. VPLEX inter-site communication communication network failure (non- communication network failure (non- network failure redundant) during regular operation redundant) during regular operation (non-redundant) without dirty data in the non- without dirty data in the non- plus inter-cluster preferred cluster.” preferred cluster.” vSphere network vMotion between the vSphere failure (non- clusters is not possible until the redundant) during inter-cluster vSphere network regular operation returns. without dirty data in both VPLEX clusters 15B. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13B. Inter-site VPLEX scenario “13B. Inter-site VPLEX network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation plus inter-cluster with dirty data in the non-preferred with dirty data in the non-preferred vSphere network cluster.” cluster.” failure (non- vMotion between the vSphere redundant) during clusters is not possible until the regular operation inter-cluster vSphere network with dirty data in returns. both VPLEX clusters 15C. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13A. Inter-site VPLEX scenario “13A. Inter-site VPLEX network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation plus inter-cluster without dirty data in the non- without dirty data in the non- vSphere network preferred cluster.” preferred cluster.” failure (non- vMotion between the vSphere redundant) during clusters is not possible until the vMotion without inter-cluster vSphere network dirty data in the returns. VMs of failed vMotions non-preferred VPLEX need to be restarted manually. cluster 15D. Inter-site Same VPLEX Geo behaviors as in Same vSphere behaviors as in VPLEX scenario “13B. Inter-site VPLEX scenario “13B. Inter-site VPLEX communication communication network failure (non- communication network failure (non- network failure redundant) during regular operation redundant) during regular operation (non-redundant) with dirty data in the non-preferred with dirty data in the non-preferred plus inter-cluster cluster.” cluster.” vSphere network vMotion between the vSphere failure (non- clusters is not possible until the redundant) during inter-cluster vSphere network vMotion with dirty returns. VMs of failed vMotions data in the non- need to be restarted manually. preferred VPLEX cluster 15E. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13A. Inter-site VPLEX scenario “13A. VPLEX inter-site network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation plus inter-cluster without dirty data in the non- without dirty data in the non- vSphere network preferred cluster.” preferred cluster.” failure (non- vMotion between the vSphere redundant) shortly clusters is not possible until the after vMotion inter-cluster vSphere network without dirty data in returns. Just vMotioned VMs need the non-preferred to be restarted manually. VPLEX cluster 15F. Inter-site VPLEX Same VPLEX Geo behaviors as in Same vSphere behaviors as in communication scenario “13B. Inter-site VPLEX scenario “13B. VPLEX inter-site network failure communication network failure (non- communication network failure (non- (non-redundant) redundant) during regular operation redundant) during regular operation plus inter-cluster with dirty data in the non-preferred with dirty data in the non-preferred vSphere network cluster.” cluster.” failure (non- vMotion between the vSphere redundant) shortly clusters is not possible until the after vMotion with inter-cluster vSphere network dirty data in the returns. Just vMotioned VMs need non-preferred VPLEX to be restarted manually. cluster

FIGS. 5 and 6 show alternative configurations for distributed storage systems that may be used in accordance with embodiments of the system described herein. In FIG. 5, a distributed storage system 200″ is shown that includes a host cluster 210′ as a distributed processing layer operating across the multiple sites 201, 202 and otherwise having elements like that discussed elsewhere herein. In FIG. 6, a distributed storage system 200′″ is shown in which the front end networks 215′ are shown operating as an external network accessed by each of the sites 201, 202 and otherwise having elements like that discussed elsewhere herein.

Once a director cluster (e.g., VPLEX Geo) has a vulnerability where one additional failure (or a limited number of additional failures) would result in Data Unavailability (DU) in a site according to the system described herein, it may start taking proactive measures to prevent this DU from happening. In other words, the system may transition towards an active/passive model. This may involve determining which site is a less vulnerable or otherwise preferred site in the event of the failure. Thereafter, the system may migrate all VMs to the less vulnerable (preferred) site (away from the more vulnerable site). If both sites are vulnerable to the same degree, the VMs are migrated to the customer's indicated preferred site. The director cluster (VPLEX) may initiate these migrations by specifying a “must” preference through an interface to a vCenter. According to various embodiments, a must preference from VPLEX to vSphere indicates on what ESX hosts VMs must be running in order to have and retain access to a specific virtual volume. A must preference from vSphere to VPLEX indicates on what directors a virtual volume must survive in order for VMs to have and retain access to it. Contrast this to a “should” preference for where the winning clusters contains the same volume image as the losing or stopped cluster. A should preference from VPLEX to vSphere indicates on what ESX hosts VMs should ideally be (this is a useful preference for VPLEX Metro). A should preference from vSphere to VPLEX indicates on what directors a virtual volume should ideally survive. The migrating of VMs to the site with access to its array improves I/O performance.

In embodiments like that discussed above, VPLEX Geo or Global, in combination with vSphere, may proactively migrate VMs to ensure that one additional failure (or a limited number of additional failures) will not cause a DU. Of course if the additional failure happens before the VMs are all moved the DU can still be encountered. Once vSphere has a vulnerability where one additional failure (or a limited number of additional failures) results in lack of resources to run all VMs in one site or to move all VMs to one site, it should notify VPLEX about its shortcomings. This may result in overruling the customer's indicated preferred site. Depending on the nature of the resource problem a should or must preference is given. Ideally, a should preference may be used when all VMs can run on one site but some resources become overcommitted based on limits in vSphere's resource allocations. A must preference may be used when VMs can no longer run on one site because of Reservations specified in vSphere's Resource Allocation. A must preference may also be used when the vSphere inter-cluster link becomes vulnerable.

In another embodiment, when both clusters contain dirty cache data, the director clusters 220 a,b may discard some of the last writes in order for the customer to carry on operations. As further discussed elsewhere herein, the director clusters 220 a,b may rollback the volume image to a write order consistent image, which means that an image from some prior point in time will be restored.

According to various embodiment of the system described herein, it may be advantageous to avoid occurrences of VM image rollback, particular in connection with addressing a situation where a last independent inter-cluster link fails during or shortly after a VM migration and causes not all data to be available on the cluster that is the target of the VM migration.

In an embodiment, the system described herein may provide for avoiding rollback of the distributed volume by writing to the distributed volume from only one cluster. By making sure that all the VMs that are accessing a certain file system, such as VMFS, vSphere, and/or NTFS for Hyper-V, on a distributed volume are in the cluster of the preferred site, it may not be necessary to rollback an image when a link failure occurs. This does mean that the system is used in an active/passive fashion on a cluster level. All VMs accessing the same distributed volume may have to be migrated together by suspending all the VMs on the source ESX host cluster, synchronizing the dirty blocks from the source director cluster to the destination director cluster, moving the cluster preference to the destination cluster, and resuming all the VMs on the destination ESX host cluster.

The following discusses a specific embodiment for avoiding rollback according to the system described herein for failure conditions like that discussed above. In the event of an expected link failure or a condition that results in a determination that link failure between a source site and the destination site is more likely, at the moment that the host cluster (e.g., a vSphere cluster) decides to start a VM migration to another site (e.g., which site may be the “winner” or preferred site in the event of a failure condition), it indicates this to the director cluster (e.g., a VPLEX cluster) at the source site by issuing an asynchronous SYNCHRONIZE CACHE command to the affected logical units of the director cluster at the source site. The host cluster, either at the source site or as a distributed layer, may start pushing memory pages to the destination ESX host. The director cluster at the source site may start pushing storage data (e.g., in one or more data blocks) for the volume groups containing the affected logical units to the destination cluster.

Once the host cluster has pushed enough pages over to enable the host cluster to suspend the VM on the source ESX host, the host cluster may issue a synchronous SYNCHRONIZE CACHE command to the affected logical units of the director cluster at the source site. The host cluster then pushes the remaining memory pages to the destination ESX host, and the director cluster at the source site pushes the remaining storage data to the destination director cluster. Once the director cluster at the source site finishes, the director cluster then completes the second (synchronous) SYNCHRONIZE CACHE command. Once the host cluster sees that both the memory pages and the storage data are available to the destination ESX host, the host cluster resumes the VM on the destination host. If the director cluster somehow takes too long before it responds to the synchronous SYNCHRONIZE CACHE command (in other words it takes too long pushing the remaining storage data to the destination cluster), the VM can be resumed on the source ESX host and the process can be restarted at an appropriate one of the above-noted steps.

Therefore, in accordance with the system described herein, the above-noted embodiment provides for synchronizing data between a source and destination site during a migration operation by moving dirty data from a source site to a destination site along with a VM that was running at the source site and then resuming the operation of the VM on the destination site.

FIG. 7 is a flow diagram 300 showing a method for providing mobility of a VM between a source site, such as site A 201, and a destination site, such as site B 202, of an active/active system according to an embodiment of the system described herein. At a step 302, for a VM operating on the source site, a determination is performed to identify the destination site that may be, for example, a preferred site and/or a winner site. Determining the destination site may occur in response to a determination that at least one additional failure will result in a lack of resources to maintain desired operation of the VM on the source site. After the step 302, processing proceeds to a step 304 where, while the VM is operating on the source site, an amount of storage data is transferred from the source site to the destination site. After the step 304, processing proceeds to a step 306 where at least a portion of the VM image is transferred to the destination site. In various embodiments, the steps 304 and 306 may be performed in a different order than shown and/or may be performed concurrently.

After the step 306, processing may proceed to a test step 308 where it is determined if a link failure has occurred between the source site and the destination site. As further discussed elsewhere herein, failure conditions may be determined using a witness node, for example. If it is determined at the test step 308 that a link failure has occurred, then processing proceeds to a step 330 where the transfer processing to the destination site is stopped and the VM is to be kept at the source site. After the step 330, processing is complete.

If it is determined that a link failure has not occurred at the test step 308, then processing proceeds to another test step 310 where it is determined whether a sufficient amount of storage data and/or memory of the VM (VM image) has been transferred to the destination site. In various embodiments, a sufficient amount of storage data and/or VM image information may be determined according to one or more thresholds. For example, a threshold may be whether 90% of the storage data and/or the VM image has been transferred. In this way, since the VM continues to operate on the source site during this part of the process, changes to storage data and/or the VM image while the VM is operating in the source site may affect the determinations of the amount of current storage data and/or the VM image that has been transferred. The threshold may be configured to establish that at a particular point in time, given the amount of storage data and VM image information transferred, it is appropriate to proceed with processing to suspend and resume the VM on the destination site, as further discussed herein.

Accordingly, if, at the test step 310, it is determined that sufficient amounts of the storage data and/or VM image have not been transferred, then processing proceeds back to the step 304. Otherwise, if it is determined that a sufficient amount has been transferred at the test step 310 according to the threshold(s), then processing proceeds to a step 312 where operation of the VM is suspended on the source site. After the step 312, processing proceeds to a step 314 where a remainder amount of the storage data, that is remaining on the source site and that had not been transferred in the prior steps, is transferred from the source site to the destination site. After the step 314, processing proceeds to a step 316 where an a remainder amount of the VM image information, that is remaining on the source site and that had not been transferred in the prior steps, is transferred from the source site to the destination site Like that of steps 304 and 306, in various embodiments, the steps 314 and 316 may be performed in an order other than that shown and/or may be performed concurrently.

After the step 316, processing proceeds to a step 318 where it is again determined if a link failure has occurred between the source site and the destination site. If it is determined at the test step 318 that a link failure has occurred, then processing proceeds to the step 330 where the transfer processing to the destination site is stopped and the VM is to be kept at the source site. After the step 330, processing is complete.

If it is determined that a link failure has not occurred at the test step 318, then processing proceeds to another test step 320 where transfer of the storage data and VM image to the destination site is completed. That is, for example, completion of data and VM image transfer may be when 100% of the storage data and VM image have been transferred. If not, then processing proceeds back to the step 314. If the transfer to the destination site is determined to be completed at the test step 320, then processing proceeds to a step 322 where the VM is resumed on the destination site according to the transferred storage data and the transferred VM image. After the step 322, processing is complete.

In various embodiments, the above-noted processing may also include provisions for error processing in which it is determined that processing, such as transferring of storage data and/or the VM image, may not be performed and/or may not be desirable to perform. For example, a system may include mechanisms for factoring in load determination processing such that a determination that load on a network is high may cause cancellation of the VM transfer even where a link failure has not occurred. Similarly, the error processing may be performed in connection with exceeding a time threshold where an attempt to transfer storage data and/or the VM image has timed out over a network. It is also noted that the above-noted method, and other methods discussed herein, may be performed using executable code stored on a non-transitory computer readable medium that is executed by one or more processors and is performed in connection with a system having components like that further discussed elsewhere herein.

Another alternative embodiment of the system described herein is to make ESX hosts aware of the changed volume state by rebooting them so they pick up the current state. This assumes that ESX hosts do not forward information about their state to vCenter or other ESX hosts. All ESX hosts that may have state about volumes that have rolled back can be put in maintenance mode. The ESX host may not have to access the volume to store its updated state, because such access would risk corrupting the VMFS. By parking an ESX host in maintenance mode, all VMs that are not talking to the rolled back storage may be migrated to other ESX hosts. Once no VMs are remaining on the ESX host, it can be rebooted and pick up the new, rolled back state of the volume.

According to another embodiment of the system described herein, the system may control rollback of a volume, or portion thereof, in a different way. For example, the system may only rollback the state of the volume to a write order consistent image on the remote cluster. For example, a winning site may contain all local writes and only the remote writes that have been synchronized locally in a write order consistent way. This would result in a volume image that is not globally consistent, and it is noted that some applications may not be able to deal with this alternate rollback state.

According to another embodiment, the system described herein may provide for the use of the inter-cluster links between the host clusters (host networks 205) and the inter-cluster link between the director clusters (director networks 225) of multiple sites to be used to help migrate a VM. Such operation may be used to increase bandwidth of the host cluster enough that it is able to migrate over longer distances. Accordingly, instead, or in addition to, the transferring of a VM image using the host networks 205 and the storage data using the director networks 225, the host networks 205 may be used to transfer storage data and/or the director networks 225 may be used to transfer VM images. For example, in an embodiment, either the VM image and/or the storage data may be transferred over the director networks 225 between the director clusters in connection with a VM migration operation. In another embodiment, either the VM image and/or the storage data may be transferred over the host networks 205. It is further noted that in connection with this operation, a host cluster may perform writes of the VM's image to the source director cluster at the source site and corresponding reads to the destination director cluster at the destination site. The above-noted method may be performed in connection with a determination of a failure of an inter-cluster link, with at least a last remaining inter-cluster link in operation. That is, even though one inter-cluster link has failed, such as failure of host networks 205, migration of a VM may be advantageously enhanced by using the remaining inter-cluster link, such as director networks 225, to help with the migrating by increasing the bandwidth of the host cluster, and vice versa.

FIG. 8A is a flow diagram 400 showing a method for using the inter-cluster link (director networks 225) between director clusters 220 a,b of multiple sites 201, 202 in an active/active system according to an embodiment of the system described herein. At a step 402, operation of a VM is initiated, or otherwise maintained, on a source site (e.g., site A 201). After the step 402, processing proceeds to a step 404 where at least some storage data is transferred to the destination site (e.g., site B 202) over the director networks 225. After the step 404, processing proceeds to a step 406 where at least a portion of a VM image is transferred to the destination site over the director networks 225. In this embodiment, the portion of the VM image may be transferred over the director networks to a shared memory (e.g. cache) location on the destination site. After the step 406, processing proceeds to a step 408 where a read of the VM image, that was transferred to the destination site, is received at the host cluster of the source site. After the step 408, processing proceeds to a step 410 where a director of the source site fetches the VM image from the shared memory location on the destination site over the director networks 225 to service the read request. Accordingly, in this embodiment the VM image and storage data may be transferred from a source site to a destination site using the director networks 225. It is further noted that the order of various steps described above may be different than that shown and/or may be performed concurrently. After the step 410, processing is complete.

FIG. 8B is a flow diagram 450 showing a method for using the inter-cluster link (host networks 205) between host clusters 210 a,b of multiple sites 201, 202 in an active/active system according to an embodiment of the system described herein. At a step 452, operation of a VM is initiated, or otherwise maintained, on a source site (e.g., site A 201). After the step 452, processing proceeds to a step 454 where at least a portion of the VM image is transferred to the destination site (e.g., site B 202) over the host networks 205. After the step 454, processing proceeds to a step 456 where at least some storage data (dirty data) is read by a host at the source site (e.g., site A 201). After the step 456, processing proceeds to a step 458 where the host at the source site sends the storage data to the destination site over the host networks 205. After the step 458, processing proceeds to a step 460 where a host at the destination site writes the storage data to the destination site. Accordingly, in this embodiment the VM image and storage data may be transferred from a source site to a destination site using the host networks 205. It is further noted that the order of various steps described above may be different than that shown and/or may be performed concurrently. After the step 460, processing is complete.

In various embodiments, the processing of FIGS. 8A and 8B may be performed in various combinations with each other. That is, the VM image and storage data may be transferred from a source site to a destination site using the host networks 205 and the director networks 225 in any appropriate combination. For example, embodiments of the system described herein may be performed in combination with normal operations using the host networks 205 and the director networks 225, that is, in which storage data is transferred over the director networks 225 (see, e.g., step 402 of FIG. 8A) and/or a VM image is transferred over the host networks 205 (see, e.g., step 452 of FIG. 8B), along with the use of the director networks 225 to transfer VM images and the use of the host networks 205 to transfer storage data.

When the inter-cluster storage network fails, the symmetric view of the data blocks may no longer be maintained over the inter-cluster storage network. Generally, the storage application stops exporting a disk image on one site. At this point, the inter-cluster computer network may still be available and may be utilized to synchronize the storage to the winning site in accordance with the system described herein. This may be done in two ways: (1) pushing disk block ownership and data from losing site to winning site and/or (2) pulling disk block ownership and data from losing site to winning site.

FIG. 9 is a flow diagram 500 showing a method of pushing data block ownership and data from losing site to winning site in the event of a director network failure according to an embodiment of the system described herein, and in which alternate networks, such as the host networks, may be used. At a step 502, an application is started on one of the computers to transfer ownership of data blocks to the winning site. After the step 502, processing proceeds to a step 504 where the application queries the losing site for ownership of clean and dirty data blocks. After the step 504, processing proceeds to a step 506 where the ownership of the clean data blocks is transferred to the winning site from the losing site. After the step 506, processing proceeds to a step 508 where the dirty data blocks and the ownership of those blocks is transferred from the losing site to the winning site. In an embodiment, the ownership of dirty data blocks may only be transferred to the winning site after the dirty blocks are synchronized to the winning site, thereby allowing the winning site director cluster to serve incoming Ms for those blocks. It is noted that the above-noted processing may be done in-band and out-of-band. After the step 508, processing is complete.

FIG. 10 is a flow diagram 550 showing a method of pulling data block ownership and data from the losing site to the winning site according to an embodiment of the system described herein. Reads and writes to the winning site may pull ownership and dirty data blocks from the losing site on an as-needed basis. At a step 552, an application is started on one of the computers to transfer ownership and dirty data blocks to the winning site when needed. At a step 554, the application issues a command to the losing site that requests ownership of data blocks and dirty data that are the subject of the command. In an embodiment, the command may be issued in connection with transmission of an error code. After the step 554, processing proceeds to a test step 556 where it is determined whether the requested data blocks are all clean data. If all the requested blocks are all clean data blocks, then processing proceeds to a step 558 where the application only receives the ownership of the data blocks. After the step 558, processing proceeds to a step 560 where the application passes the ownership of the data blocks to the winning site. The winning site can provide the clean data blocks after receiving the ownership. After the step 560, processing is complete. If, at the test step 556, it is determined that at least some of the requested blocks are dirty, then processing proceeds to a step 562 where the application receives the dirty blocks and the ownership. After the step 562, processing proceeds to a step 564 where the dirty blocks and the ownership are passed to the winning site. After the winning site receives the ownership, it can start serving I/Os for these blocks and no longer needs to pass an error code back. After the step 564, processing is complete.

A combination of the methods of pushing and pulling disk storage data and ownership is also possible according to various embodiments of the system described herein. For example, the combination of pushing the storage block ownership of clean blocks and pulling the block ownership and data of dirty blocks may be desirable.

According further to the system described herein, when the last redundant resource of a cluster level vulnerability fails, one of the clusters will no longer serve I/O, either because the cluster failed or because cache coherence can no longer be guaranteed between the clusters. If all VMs made it in time to safety on the less vulnerable or preferred site that is still serving I/O as set forth herein, then the system may continue operating in the manner provided. If not, however, three further options may be provided according to various embodiments of the system described herein:

1. All VMs may be restarted from a Write Order Fidelity (WOF) consistent virtual volume (e.g., from a couple of seconds ago). Due to the restart, the VMs may boot with an empty memory state.

2. All VMs may be restarted with an application consistent snapshot of the virtual volume. Periodic snapshot of the virtual volume may be created to facilitate this approach.

3. All VMs may be resumed with a memory state and virtual volume state from a coordinated VM/disk snapshot. Periodic coordinated snapshots of both VM (with memory state) and virtual volume may be created to facilitate this approach.

In case 1, all applications running in VMs should be able to detect what work is lost by studying the current state of the disk and carry on from that point. In case 2, all applications running in VMs may simply carry on without having to check the state of the disk. In cases 2 and 3, all applications running in VMs may redo the work from the moment that the snapshots were taken. Note that external transactions to the VMs that occurred since the snapshot may be lost.

FIG. 11 is a schematic illustration of a write order fidelity (WOF) pipeline 600 and visualizes some details that may be used according to an embodiment of the system described herein. Write order consistency may be achieved by bucketing writes in “deltas” and providing consistency on a delta boundary basis. Further details of WOF may be found in U.S. Pat. No. 7,475,207 to Bromling et al., which is referenced elsewhere herein. Deltas may have ids that are monotonically increasing, where older deltas have lower numbers. The WOF pipeline may include multiple deltas.

FIG. 11 shows deltas 2 through 6. The contents of delta 1 made it to the distributed volume as can be seen by “one” and “uno” being present on the disks 620. The director cluster may have an open delta with id 6. Hosts 610 in each site are writing “six” on Site A, and “seis” on Site B to the open delta. At some point the open delta closes indicating that no new writes are accepted while outstanding writes are finishing their data phase. The closing of the delta may be coordinated between all directors in both clusters. Once the last write for the closing delta is finished the closing delta is closed and a new delta (with id 7) is opened and new writes will be accepted into the new delta. The blocks in closed deltas may be exchanged at a later time. In the figure, delta 4 is in the exchange phase as can be seen by “four” being exchanged to Site B and “cuatro” being exchanged to Site A. Once a delta is fully exchanged, the next available closed delta is exchanged. Sometime later, exchanged deltas are committed to disk. In the figure, the delta with id 2 is committing, as can be seen by both sites writing “two” and “dos” to their local leg of the distributed volume. Note that both legs of the distributed volume may be substantially identical (modulo currently committing deltas) even though some writes were initiated by hosts at Site A and others by hosts at Site B. In an embodiment herein, a point at which deltas close may be referred to as a “check point” and/or a “system check point”. In response to a failure, the system may roll back to a most recent check point. In some embodiments, generating a system check point at a site causes a corresponding failover site to pull dependent data (e.g., data write) thereto where the failover site requests the missing data.

FIG. 12 is a flow diagram 700 showing a method for taking and using synchronized snapshots of the memory state (image) of a VM and the storage state of a disk to provide a fail-over mechanism for high availability. At a step 702, a determination is made to trigger a check point in which a synchronized VM image snapshot and storage state snapshot of a disk are obtained. For example, in an embodiment, triggering a check point may occur following a failure condition in which one additional failure may result in insufficient resources for operation of the VM on an active/active system, as further discussed elsewhere herein. The failure may include a link failure and/or a failure of a site. In another embodiment, a user may trigger a check point. In still other embodiments, check points may be triggered by a SCSI command, a user command, and/or an I/O operation.

After the step 702, processing proceeds to a step 704 where the VM is suspended such that a snapshot image of the VM may be obtained. After the step 704, processing proceeds to a step 706 where I/O processing is suspended such that a storage state snapshot may be obtained. Various techniques may be used in connection with suspending I/O processing. It is noted that steps 704 and 706 may be reversed, as appropriate, and performed in parallel.

After the step 706, processing proceeds to a step 708 where a snapshot image of the VM is taken when the VM is preparing to commit a write of a delta to a bucket in accordance with WOF pipeline processing as further discussed elsewhere herein. After the step 708, processing proceeds to a step 710 where a snapshot is taken of the storage state of a disk being utilized to store data in connection with the WOF transfer processing. The VM image snapshot and the storage state snapshots are synchronized and it is noted that the order of steps 708 and 710 may be reversed, as appropriate, and/or may be performed in parallel. After the step 710, processing proceeds to a step 712 where the VM is resumed. After the step 712, processing proceeds to a step 714 where I/O processing is resumed. It is noted that the steps 712 and 714 may be reversed, as appropriate, or performed in parallel. After the step 714, processing proceeds to a step 716 where the VM image snapshot and storage state snapshot are made available for subsequent access as further discussed elsewhere herein. After the step 716, processing is complete.

FIG. 13 is a flow diagram 750 showing processing following a failure in the active/active system according to an embodiment of the system described herein. At a step 752, a determination is made that a failure has occurred in the active/active system, such as a link failure between multiple sites and/or a failure at one of the sites. After the step 752, processing proceeds to a step 754 where a rollback determination is made to rollback the state of a VM and disk array to a prior state (i.e., the most recent check point). After the step 754, processing proceeds to a step 756 where the VM is suspended. After the step 756, processing proceeds to a step 758 where I/O processing is suspended. In various embodiments, the order of steps 756 and 758 may be reversed, as appropriate, and/or performed in parallel. After the step 758, processing proceeds to a step 760, where the VM is rolled back to prior check point, which corresponds to a prior state of operation in connection with the VM image snapshot. After the step 760, processing proceeds to a step 762 where disk array is rolled back a prior storage state (i.e., the most recent check point). Specifically, the VM and storage state of the disk may be rolled back to a state corresponding to the snapshots taken in connection with the flow diagram 700. In this way, a VM and disk state may be advantageously rolled back to an incremental delta state that avoids data loss by using one or more of the incremental delta snapshots taken during the WOF processing in connection with the committing of write deltas to buckets according to the system described herein. The incremental delta snapshot used for the rollback may correspond to a time just prior to the failure that triggered the rollback. It is noted that steps 760 and 762 may be performed in a different order than that shown and/or may be performed concurrently.

After the step 762, processing proceeds to a step 764 where the VM is resumed according to the rollback state snapshots from the most recent check point. After the step 764, processing proceeds to a step 766 where I/O processing is resumed. In various embodiments, the order of steps 764 and 766 may be reversed, as appropriate, and/or performed in parallel. After the step 764, processing is complete.

In some cases, applications provide their own check point and roll back mechanism that is independent of the VM and storage mechanism (system check points) described elsewhere herein. The mechanisms provided by applications (application check points) may be especially suited to the specific applications. For example, a bank application may transfer money from one user account to another. The application may provide a first application check point just prior to the start of the first transfer operation and then a second application check point after the transaction completes. If a failure occurs while the transaction is in process, the application may roll back to the first application check point. In that way, the application avoids inconsistencies such as a situation where funds are debited from one of the user's accounts without being credited to the other one of the user's accounts. Note, however, that even if the transaction completes and the bank application provides the second application check point, the system may roll back to a system check point that is prior to the second application check point. In such a case, following the system rolling back to the system check point, the application would then need to roll back to the first application check point, thus undoing the transaction that had otherwise completed prior to the failure. It is desirable to avoid this.

Referring to FIG. 14, a flow diagram 800 illustrates steps performed in connection with coordinating an application check point with system check points. The processing illustrated by the flow diagram 800 may be performed whenever an application provides an application check point. Processing begins at the first step 802 where the application check point is performed. Performing the application check point at the step 802 is application specific and the processing therefor depends upon the application. Following the step 802 is a step 804 where a system check point is triggered, as discussed in more detail elsewhere herein. See, for example, the discussion provided in connection with the step 702 in the flow diagram 700, discussed above.

Following the step 804 is a test step 806 where it is determined if the system check point operation is complete (e.g., whether all of the processing illustrated by the flow diagram 700, discussed above, has been performed). If so, then processing is complete. Otherwise, the system continues to poll (loop back to the test step 806). An application that performs an application check point performs processing like that illustrated by the flow diagram 800. Thus, the application check point process is not complete until after the step 806. That is, the application check point does not complete until the system check point also completes.

Note that the processing illustrated by the flow diagram 800 may be performed in addition to, and/or instead of, any other processing that may be performed in connection with triggering system check points. Thus, for example, if the system otherwise regularly performs system check points every fifteen minutes, the processing illustrated by the flow diagram 800 may be performed 1) instead of the regular system check point processing; 2) in addition to the regular check point processing; or 3) in addition to the regular check point processing, except that the regular check point processing is skipped when the processing illustrated by the flow diagram 800 is performed near the time of a regular system check point time.

Referring to FIG. 15, a flow diagram 810 illustrates steps performed in connection with an alternative embodiment for coordinating an application check point with system check points. The processing illustrated by the flow diagram 810 may be performed whenever an application provides an application check point. Processing begins at the first step 812 where the application check point is performed. Performing the application check point at the step 812 is application specific and the processing therefor depends upon the application. Following the step 812 is a step 814 where a system check point is triggered, as discussed in more detail elsewhere herein. See, for example, the discussion provided in connection with the step 702 in the flow diagram 700. Following the step 814, processing is complete.

Note that, unlike the processing illustrated by the flow diagram 800, the processing illustrated by the flow diagram 810 does not ensure that the system check point is complete following an application check point. Instead, the system check point process is only started (e.g., triggered at the step 702, discussed elsewhere herein). The processing illustrated by flow diagram 810 is advantageous in situations where in may be unacceptable to wait for a system check point to complete each time an application check point is provided.

Note also that, just as with the processing illustrated by the flow diagram 800, the processing illustrated by the flow diagram 810 may be performed in addition to, and/or instead of, any other processing that may be performed in connection with triggering system check points. Thus, for example, if the system otherwise regularly performs system check points every fifteen minutes, the processing illustrated by the flow diagram 810 may be performed 1) instead of the regular system check point processing; 2) in addition to the regular check point processing; or 3) in addition to the regular check point processing, except that the regular check point processing is skipped when the processing illustrated by the flow chart 800 is performed near the time of a regular system check point time.

In some embodiments, it may be useful to provide a first set of one or more data centers that are relatively active (primary data centers) and a second set of one or more data centers that are relatively inactive (failover data centers). The first set of data centers and second set of data centers may both be used for application reading and writing, but the first set of data centers may be more active and/or include more response time sensitive applications than the second set of data centers. Each of the relatively active data centers in the first set of data centers may use at least one corresponding data center in the second set of data centers for failover operations. For simplification, the discussion herein will show a single relatively active data center and a single relatively inactive data center. However, it should be understood that the system described herein may use more than one relatively active data centers and/or more than one relatively inactive data center.

FIG. 16 is a schematic illustration showing a distributed storage system 900 having a first, relatively active, primary site 902 a and a second, relatively inactive, failover site 902 b. The sites 902 a, 902 b may include one or more hosts grouped in host clusters 910 a, 910 b, one or more directors grouped in director clusters 920 a, 920 b, and disk arrays 940 a, 940 b. Each of the host clusters 910 a, 910 b and the director clusters 920 a, 920 b may each include software and/or other controllers or interfaces to control or administer operations in connection with described functions of the hosts and directors. In an embodiment, each of the host clusters 210 a, 210 b may include ESX hosts in a vSphere cluster and the director clusters 920 a, 920 b may include directors in a VPLEX cluster.

In an embodiment herein, failure of the primary site 902 a causes a failover to the failover site 902 b. Applications that were running on the primary site 902 a are transferred and made to run on the failover site 902 b. A system check point rollback is performed at the failover site 902 b followed by each of the applications at the failover site 902 b performing an application-specific rollback which, as discussed elsewhere herein, should be to the latest application specific check point issued by each of the applications.

Each of the sites 902 a, 902 b includes front end networks 915 a, 915 b that may connect through host links to the host clusters 910 a, 910 b and through front end links to the director clusters 920 a, 920 b. One or more back end networks 930 a, 930 b may connect through back end links to, the director clusters 920 a, 920 b and through array links to the disk arrays 940 a, 940 b. In an embodiment, the front and back end networks may be Fibre Channel networks. The front end networks 915 a, 915 b allow the hosts (or VMs running therein) to perform I/O operations with the host clusters 910 a, 910 b, while the back end networks 930 a, 930 b allow the directors of the director clusters 920 a, 920 b to perform I/O on the disk arrays 940 a, 940 b. A host network 905, such as a vSphere Ethernet network, may connect the ESX hosts in the host clusters 910 a, 910 b. A director network 225 may connect the directors of the director clusters 920 a, 920 b.

In an embodiment, VPLEX Geo may be used and configured to expose one or more distributed volumes from both VPLEX director clusters. A VMFS may be created on top of these distributed volumes allowing VMs that migrate between the sites to see the same file system in either site. It is also noted that, as illustrated and according to various embodiments, The sites 902 a, 902 b may include redundancies in hosts, directors and links therebetween.

The sites 902 a, 902 b may be separated by an asynchronous distance (i.e., a distance such that asynchronous communications between the sites is desirable). A write operation to cluster at a remote site may be acknowledged as soon as a protection copy is made within the cluster. Sometime later the write data is acknowledged by the remote site. Similarly, writes to the remote site are later synchronized to a cluster at the local site. Software or other controllers at the director clusters, such as VPLEX, may present the same image of the data on either cluster to provide a cache-coherent view of the data. In an embodiment, this may be achieved by fetching data that has not yet been replicated between a source and destination site (i.e. “dirty” data; as compared with “clean” data which has been copied and is protected on multiple sites) over the inter-cluster link on an as needed basis. In the background, the controller (VPLEX) may synchronize the oldest dirty data between the clusters.

In an embodiment herein, the primary site 902 a is relatively active and/or is running applications that are relatively time sensitive. In such a case, it may be impractical to provide a level of synchronization like that illustrated above in connection with the flow diagram 800 of FIG. 14 since it may not be acceptable to wait for a system check point to complete after each application check point. Accordingly, the primary site 902 a may provide system check points without regard to when application check points are provided and/or may provide system check points in accordance with the processing illustrated in connection with the flow diagram 810 of FIG. 15. However, even if the primary site 902 a is relatively active and/or is running applications that are relatively time sensitive, it is possible for the failover site 902 b to be relatively inactive or to otherwise be able to coordinate application check points with system check points as illustrated by the flow diagram 800 of FIG. 14.

As an example, the primary site 902 a may be running an automated stock trading program where dozens of stock transactions are automatically executed every second. The failover site 902 b may only process user transactions which are much less frequent than the automated transactions (e.g., user-initiated on-line trading transactions). Each of the transactions at the site 902 a may result in an application check point, but for practical reasons it may not be acceptable to require completion of a system check point following each of the application check points at the site 902 a. Thus, the site 902 a may perform processing like that illustrated by the flow chart 810 or possibly even without coordinating application check points and system check points at all. However, it may be acceptable for the other site 902 b to complete system check points following providing application check points, as illustrated by the flow diagram 800 of FIG. 14. Thus, the sites may be configured asymmetrically with respect to coordination between application check points and system check points.

In an embodiment herein, I/O operations performed at the failover site 902 b (if any) may be performed synchronously so that a write at the failover site 902 b is not acknowledged unless and until the data that was written at the failover site 902 b is transferred to the primary site 902 a and/or possibly until data on which the latest I/O at he failover site is dependent upon is pulled from the primary site 902 a.

Note that, generally, it is possible for each of the sites 902 a, 902 b to handle application check points differently depending upon any criteria. For example, the site 902 a may have a first set of applications for which a system check point is completed after each application check point and a second set of applications for which a system check point is not immediately completed following an application check point. Similarly, the site 902 b may have a first set of applications for which a system check point is completed after each application check point and a second set of applications for which a system check point is not immediately completed following an application check point. Note also that any appropriate criteria may be used for determining whether to complete a system check point following an application check point. For example, it may be possible to monitor dynamic system performance and to causes a system check point to immediately follow an application check point when the system activity level is relatively low. Other criteria can include specific volumes and/or groups of volumes. The criteria may be manual or automatic or some combination thereof. The criteria may be asymmetrical with respect to the sites 902 a, 902 b.

Referring to FIG. 17, a flow diagram 960 illustrates processing performed in connection with selectively causing system check points to immediately follow application check points. The processing illustrated by the flow diagram 960 may be performed whenever an application intends to perform an application check point. Processing begins at a first step 962 where the application check point is performed. Performing the application check point at the step 962 is application specific and the processing therefor depends upon the application. Following the step 962 is a step 964 where a system check point is triggered, as discussed in more detail elsewhere herein. See, for example, the discussion provided in connection with the step 702 in the flow diagram 700. Following the step 964 is a test step 966 where it is determined if whatever criteria (discussed elsewhere herein) is being used to cause a system check point to immediately follow an application check point is met. If not, then processing is complete. Otherwise, control transfers to a test step 968 where it is determined if the system check point operation is complete (e.g., whether all of the processing illustrated by the flow diagram 700, discussed above, has been performed). If so, then processing is complete. Otherwise, the system continues to poll (loop back to the test step 968). Note that, just as with the flow diagram 800, the processing illustrated by the flow diagram 960 may be performed in addition to, and/or instead of, any other processing that may be performed in connection with triggering system check points.

Various embodiments discussed herein may be combined with each other in appropriate combinations in connection with the system described herein. Additionally, in some instances, the order of steps in the flowcharts, flow diagrams and/or described flow processing may be modified, where appropriate. Further, various aspects of the system described herein may be implemented using software, hardware, a combination of software and hardware and/or other computer-implemented modules or devices having the described features and performing the described functions. Software implementations of the system described herein may include executable code that is stored in a computer readable medium and executed by one or more processors. The computer readable medium may include a computer hard drive, ROM, RAM, flash memory, portable computer storage media such as a CD-ROM, a DVD-ROM, a flash drive and/or other drive with, for example, a universal serial bus (USB) interface, and/or any other appropriate tangible or non-transitory computer readable medium or computer memory on which executable code may be stored and executed by a processor. The system described herein may be used in connection with any appropriate operating system.

Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. A method of maintaining data at a failover site, comprising: an application writing data at a primary site corresponding to the failover site, the primary site and the failover site being part of a storage system that controls transfer of data asynchronously from the primary site to the failover site; the application issuing an application check point, the application check point being specific to the application; and initiating a system check point in response to the application issuing the application check point, the system check point being initiated by the storage system and being different from the application check point issued by the application, wherein the system check point causes data at the failover site to be consistent with the primary site.
 2. The method, according to claim 1, wherein the application check point completes only after the system check point completes.
 3. The method, according to claim 1, wherein the application check point completes independently of completion of the system check point.
 4. The method, according to claim 1, wherein the application check point completes either only after the system check point completes or independently of completion of the system check point according to a specific criteria.
 5. The method, according to claim 1, wherein data for I/O operations performed at the failover site is transferred synchronously from the failover site to the primary site.
 6. The method, according to claim 1, further comprising: regularly initiating system check points independently of any application check points.
 7. The method, according to claim 6, further comprising: skipping a regularly scheduled system check point in response to having recently performed a system check point following an application check point.
 8. The method, according to claim 1, wherein each of the sites includes at least one host, at least one director, and at least one disk array.
 9. The method, according to claim 1, wherein the system check point includes providing dependent cache data from the primary site to the failover site.
 10. The method, according to claim 9, wherein the data is pulled from the primary site by the failover site.
 11. A non-transitory computer-readable medium storing software that maintains data at a failover site when an application writes data at a primary site corresponding to the failover site, the primary site and the failover site being part of a storage system that controls transfer of data asynchronously from the primary site to the failover site, the software comprising: executable code that determines when the application issues an application check point, the application check point being specific to the application; and executable code that initiates a system check point in response to the application issuing the application check point, the system check point being initiated by the storage system and being different from the application check point issued by the application, wherein the system check point causes data at the failover site to be consistent with the primary site.
 12. The non-transitory computer readable medium according to claim 11, wherein the application check point completes only after the system check point completes.
 13. The non-transitory computer readable medium according to claim 11, wherein the application check point completes independently of completion of the system check point.
 14. The non-transitory computer readable medium according to claim 11, wherein the application check point completes either only after the system check point completes or independently of completion of the system check point according to a specific criteria.
 15. The non-transitory computer readable medium according to claim 11, wherein data for I/O operations performed at the failover site is transferred synchronously from the failover site to the primary site.
 16. The non-transitory computer readable medium according to claim 11, wherein the software further comprises: executable code that regularly initiates system check points independently of any application check points.
 17. The non-transitory computer readable medium according to claim 16, wherein the software further comprises: executable code that skips a regularly scheduled system check point in response to having recently performed a system check point following an application check point.
 18. The non-transitory computer readable medium according to claim 11, wherein each of the sites includes at least one host, at least one director, and at least one disk array.
 19. The non-transitory computer readable medium according to claim 11, wherein the system check point includes providing dependent cache data from the primary site to the failover site.
 20. The non-transitory computer readable medium according to claim 19, wherein the data is pulled from the primary site by the failover site. 